Handle commits and tags without messages

Annotated tags don't necessarily have messages, and Git even tolerates
commits without messages. So don't insist that the headers for such
objects be followed by a blank line.

Fixes #41.

Author: mhagger

git/git.go

@@ -569,13 +569,24 @@ type ObjectHeaderIter struct {
 	data string
 }
 
-// Iterate over an object header. `data` should be the object's
-// contents, including the "\n\n" that separates the header from the
-// rest of the contents. `name` is used in error messages.
+// Iterate over a commit or tag object header. `data` should be the
+// object's contents, which is usually terminated by a blank line that
+// separates the header from the comment. However, annotated tags
+// don't always include comments, and Git even tolerates commits
+// without comments, so don't insist on a blank line. `name` is used
+// in error messages.
 func NewObjectHeaderIter(name string, data []byte) (ObjectHeaderIter, error) {
 	headerEnd := bytes.Index(data, []byte("\n\n"))
 	if headerEnd == -1 {
-		return ObjectHeaderIter{}, fmt.Errorf("%s has no header separator", name)
+		if len(data) == 0 {
+			return ObjectHeaderIter{}, fmt.Errorf("%s has zero length", name)
+		}
+
+		if data[len(data)-1] != '\n' {
+			return ObjectHeaderIter{}, fmt.Errorf("%s has no terminating LF", name)
+		}
+
+		return ObjectHeaderIter{name, string(data)}, nil
 	}
 	return ObjectHeaderIter{name, string(data[:headerEnd+1])}, nil
 }